As used herein, the term "fly ash" refers to the ash produced by and from the the combustion of powdered or other particulate forms of coal, such as in power station boilers or the like, and includes entrained ash from a gassifier. The term "scrubber sludge" includes the solid materials recovered from the combustion gases of coal, through conventional means such as smoke stacks, scrubbers and the like. Chemically, fly ash and scrubber sludge comprise very similar materials, except that scrubber sludge generally has a considerable amount of calcium sulfate in it, as a result of the limestone slurry typically used to react with sulfur dioxide in the gases. For the descriptions herein, "fly ash" and "scrubber sludge" will be understood to be interchangeable terms.
Fly ash and scrubber sludge are produced in great amounts almost any time coal is burned as a fuel, for example in thermoelectric power plants. Disposal of this material has been a considerable problem, and promises to be an ever increasing problem as a result of environmental restrictions, increasing costs of transport, the need for dumping sites and similar concerns. Generally, ash or sludge materials are wastes which in the past have had very little, if any, value or use, thus contributing to making disposal an economic burden and problem.
It has long been recognized that fly ash and scrubber sludge contain many potentially valuable mineral values. For example, typical fly ash includes a considerable amount of aluminum, iron, manganese, calcium, magnesium, titanium and potassium oxides therein. Further, small amounts of barium, cobalt, chromium, copper, gallium, nickel, lead, rubidium, strontium, zinc, zirconium, and other compounds have been found in fly ash. Typically, the most prevalent of these minerals is the aluminum value, which is sufficiently high to encourage the development of fly ash processing.
Numerous types of fly ash processing procedures have been developed, see for example McDowell et al. U.S. Pat. No. 4,252,777; Torma, U.S. Pat. No. 4,242,313; Murtha, U.S. Pat. No. 4,397,822; Mitchell et al., U.S. Pat. No. 3,393,975; Ashworth et al., U.S. Pat. No. 4,652,433; and British Patent 369,268, the disclosures of which are incorporated herein by reference. These and other processes generally focus attention on the recovery of alumina (Al.sub.2 O.sub.3) from the coal waste product.
Conventional methods of processing fly ash have generally not been satisfactory for several reasons. First, the alumina isolated is often not of sufficiently high purity to have much value or use as a commercial product, without substantial further, potentially expensive, processing. Thus, the conventional processes have simply not been economically feasible. Secondly, a problem has generally remained as to the disposal of the bulk of the waste material, typically various silicates. This material still would have to be disposed of, as conventional methods of processing have not provided for the generation of good commercial products from this waste, i.e., products which have both a substantial use and a significant market value.
With respect to this latter problem, it is noted that it is not enough to simply develop a side product which arguably has a use. The reason is that unless the use is substantial, the market in that side product would rapidly be saturated by the very large amounts of compound generated from the processing of the millions of tons of fly ash produced from coal plants in the United States.
Another problem with conventional methods of processing fly ash is generally related to the problem of costs in large scale operations. Many methods require substantial calcining or kilning steps on large volumes of material. Such steps are energy intensive, and are thus relatively expensive. Further, processing may itself generate undesired waste products, such as waste gases or contaminated water or washing solutions. Unless the process either avoids these products, or provides a method for recapture or recycling, the process may not be economically feasible, or otherwise desireable. That is, the processing could end up expensive and without sufficient benefit to be worthwhile.
What has been needed, and has generally been lacking, has been a method of fly ash and scrubber sludge processing which accomplishes and/or provides the following:
1. Effective and efficient recovery of valuable mineral values, especially iron and aluminum values, in useful and desireable forms, and with desired purity. PA1 2. A process which substantially reduces the volume of waste material to be dumped. PA1 3. A process whereby the bulk, in terms of volume and weight, of the fly ash material is isolated as a useful product; and one potentially having reasonable market value and sufficient utility so as to not completely glut the market in that product, if produced in the very large scale quantities possible as a result of large scale fly ash processing. PA1 4. A process that uses relatively high temperature kiln or calcining operations to a lesser extent than conventional processes, to avoid excessive energy demands. And, PA1 5. A process which produces relatively little waste or product disposal or contamination problem.